The interfacial profiles and interfacial tensions of phase-separated binary mixtures of Bose-Einstein condensates are studied theoretically. The two condensates are characterized by their respective healing lengths ${\ensuremath{\xi}}_{1}$ and ${\ensuremath{\xi}}_{2}$ and by the interspecies repulsive interaction
K. An exact solution to the Gross-Pitaevskii (GP) equations is obtained for the special case ${\ensuremath{\xi}}_{2}/{\ensuremath{\xi}}_{1}=1/2$ and
K=3/2. Furthermore, applying a double-parabola approximation (DPA) to the energy density featured in GP theory allows us to define a DPA model, which is much simpler to handle than GP theory but nevertheless still captures the main physics. In particular, a compact analytic expression for the interfacial tension is derived that is useful for all ${\ensuremath{\xi}}_{1},\phantom{\rule{0.28em}{0ex}}{\ensuremath{\xi}}_{2}$, and
K. An application to wetting phenomena is presented for condensates adsorbed at an optical wall. The wetting phase boundary obtained within the DPA model nearly coincides with the exact one in GP theory.
In this study, we determine the interface position of a binary Bose-Einstein condensate mixture in the prewetting phase, confined by a hard wall, at absolute zero temperature. An approximate analytical result is obtained by employing the double-parabola approximation (DPA) to the quartic potential energy density within the Gross-Pitaevskii (GP) theory. The interface position depends on the control parameter and the chemical potential ratio. Our analytical result shows good agreement with numerical results, which indicates that the DPA is a reliable method. This study provides insights for experiments focused on observing the wetting transition of gases in the Bose-Einstein condensate phase.
Abstract We develop the Quadruple-Parabola Approximation (QPA), an analytical, piecewise-quadratic trial-function model for the static, planar, zero-temperature interface of a phase-separated binary Bose--Einstein condensate (BEC) mixture with unequal healing lengths, $\xii_1\neq\xii_2$. The construction extends the Double- and Triple-Parabola Approximations (DPA, TPA) by assigning independent curvatures and matching positions to the two components whilepreserving exact linear solvability. The DPA, TPA, and QPA trial families are nested by construction, so that evaluating every trial profile in the exact Gross--Pitaevskii (GP) functional establishes the variational ordering $\si_{\rm DPA}\ge\si_{\rm TPA}\ge\si_{\rm QPA}\ge\si_{\rm GP}$ for arbitrary $\xii$; the QPA reproduces the TPAexactly in the symmetric limit $\xii=1$ and obeys an exact species-exchange covariance, $\si(K,\xii)=\xii\,\si(K,1/\xii)$, both verified numerically. Benchmarked against an independent GP boundary-value solver over
K∈{1.2,1.5,2,3,5} and $\xii\in\{0.5,0.7,1,1/0.7,2\}$, the QPA systematically lowers the variational interfacial tension relative to the TPA in every asymmetric case tested -- removing up to
∼30% of the TPA's residual tension error in the weakest-segregation, most-asymmetric regime, though typically only a few percent elsewhere -- while the DPA over-estimates the tension by
9--
42% against
6.0--
9.6% for the TPA and QPA. This energy improvement does not translate into a smaller pointwise profile error: with a properly
L-independent
L2 metric, the QPA profile error exceeds the TPA's in the majority of (species,
K, $\xii$) combinations tested, but the TPA is not universally superior -- in
8 of
40 such combinations, concentrated at
K≥2 and related in mirrored pairs by the species-exchange covariance, the QPA is marginally better instead. Within the parameter window, optimisation protocol, and admissible domain explored here, neither the TPA nor the QPA reaches sub-percent tension accuracy. The results clarify both the usefulness and the intrinsic limitations of piecewise-quadratic interfacial models for binary quantum fluids: alarger variational trial family can be a genuinely better energetic approximation without being a better pointwise one.